Polymer films having passages extending in their thickness direction are used in various applications. An example of the applications is the use in a waterproof sound-permeable membrane having both waterproofness and sound permeability.
A waterproof sound-permeable membrane is used, for example, in an electronic device having an audio function. Nowadays, it is typical for electronic devices such as mobile phones, tablet computers, digital cameras, and game consoles to have an audio function. The housing of such an electronic device having an audio function encloses an audio part including a sound emitter such as a speaker and/or a sound receiver such as a microphone. The housing of the electronic device typically has an opening positioned in register with the audio part. This opening allows sound to be transmitted between the outside of the electronic device and the audio part.
Naturally, entry of water into the housing of an electronic device must be prevented; however, the above opening for transmission of sound may act as a passage that permits water to easily enter the housing. In particular, portable electronic devices have an increased risk of suffering from entry of water because they are often exposed to rain or water used in daily life and because the orientation of the opening cannot be fixed at a given orientation that allows the avoidance of water (for example, a downward orientation for which rain is less likely to come into the housing). For this reason, a waterproof sound-permeable membrane that permits transmission of sound between the audio part and the outside of the housing and that also prevents water from entering the housing from the outside through the opening is placed to cover the opening.
Another example of the applications of the polymer films is the use in a waterproof gas-permeable membrane having both waterproofness and gas permeability.
Various housings are often provided with openings for maintaining ventilation between the outside and inside of the housings, and examples of the housings include: a housing enclosing an electronic circuit board such as a vehicle ECU (Electrical Control Unit) and a control board for a solar cell; a housing enclosing an electronic device or components such as a motor, a light source, and a sensor; a housing of a household electric appliance such as an electric toothbrush and an electric shaver; and a housing of an information terminal such as a mobile phone. The provision of an opening to a housing makes possible, for example, elimination or reduction of pressure difference occurring between the inside and outside of the housing. The opening is often covered by a waterproof gas-permeable membrane that has both gas permeability and waterproofness and that prevents water from entering the housing from outside through the opening while permitting permeation of gas (typically air) between the inside and outside of the housing for maintenance of ventilation, especially when a product enclosed in the housing is vulnerable to water.
Examples of the waterproof sound-permeable membrane and the waterproof gas-permeable membrane include stretched porous membranes having a structure in which a huge number of pores formed by stretching are distributed. Patent Literature 1 discloses a waterproof sound-permeable membrane including a stretched porous membrane of polytetrafluoroethylene (PTFE) or of ultrahigh molecular weight polyethylene (UHMWPE), and Patent Literature 2 discloses a waterproof gas-permeable membrane including a stretched porous membrane of PTFE. Another example of the waterproof sound-permeable membrane and waterproof gas-permeable membrane is a non-porous polymer film having through holes formed to extend through the thickness of the film (see Patent Literature 3 and 4). The waterproof sound-permeable membrane of Patent Literature 3 and the waterproof gas-permeable membrane of Patent Literature 4 are formed by irradiating a non-porous polymer film with an ion beam and then chemically etching the film.
Another example of the applications of the polymer films having passages in their thickness direction is the use in a suction sheet, which is placed on a suction face of a suction unit when a workpiece is held by suction on the suction unit. Placing the suction sheet on the suction face allows the workpiece to be held by suction on the suction unit without direct contact between the suction face and the workpiece. The workpiece is, for example, a dielectric thin film (ceramic green sheet) used for production of a ceramic capacitor.
The ceramic green sheet is fabricated by applying a dielectric paste onto a release sheet to form a coating film and drying the coating film. The fabricated ceramic green sheet lying on the release sheet serving as a support is supplied to the production process of a ceramic capacitor together with the release sheet. The ceramic green sheet supplied is separated from the release sheet, optionally after undergoing formation of an electrode film and/or cutting, and is transferred to a predetermined place where the ceramic green sheet is stacked on another ceramic green sheet. The stack of the ceramic green sheets is sintered to produce a ceramic capacitor.
The separation of the ceramic green sheet from the release sheet and the transfer of the separated ceramic green sheet are accomplished using a suction unit that holds the ceramic green sheet by suction. Such transfer using a suction unit may be called “suction transfer”. The suction transfer enables reliable separation and transfer, and precise stacking, of ceramic green sheets. The suction unit is typically made of metal, and its suction face is susceptible to flaws caused by a fine ceramic powder contained in ceramic green sheets. The flaws on the suction face act as a cause of flaws on other ceramic green sheets to be subsequently held on the suction face, and the flaws on the sheets lead to defects of the resulting ceramic capacitor. Placing the suction sheet on the suction face provides protection of the suction face, thereby reducing the occurrence of defects of the resulting ceramic capacitor. In addition, the maintenance of a suction transfer device or ceramic green sheet stacking device including the suction unit becomes easy.
An example of the suction sheet is a porous sheet including fine UHMWPE particles sintered together (see Patent Literature 5). Another example of the suction sheet is a non-porous polymer film having through holes formed to extend through the thickness of the film (see Patent Literature 6). These suction sheets have high gas permeability, good surface smoothness, and good releasing properties.
Polymer films having passages extending in their thickness direction are expected to be used in other various applications for which the properties of the films are appropriate, as well as in the applications described above.